The present invention relates generally to a fuel injection system, and more particularly, to a fuel injection system for controlling fuel delivery from a fuel injection pump that delivers fuel synchronously with rotation of an internal-combustion engine.
In conventional fuel injection systems, such as disclosed in JP-A No. S59-65523 (U.S. Pat. No. 4,492,534), a fuel metering valve is provided in a fuel feed line. The fuel feed line is positioned between a fuel feed pump, for drawing fuel from a fuel tank, and a fuel injection pump. The fuel metering valve is regularly opened and closed for metering the fuel. The amount of time this valve is opened and closed is controlled to thereby meter the quantity of fuel drawn into the fuel injection pump.
In operation, however, the quantity of fuel drawn increases relative to change in the valve opening timing since the fuel metering valve is operated from a full-closed position to a wide-open position or vice versa. Therefore, in the conventional fuel injection system, the quantity of fuel altered by changing the valve opening timing of the fuel metering valve increases with change. Therefore, it is impossible to precisely control fuel quantity drawn into the fuel injection pump.
The present applicant, therefore, changed the fuel metering valve control from that described above to controlling the opening area of the valve to precisely control the quantity of fuel drawn into the fuel injection pump (and accordingly the amount of fuel delivery from the fuel injection pump) JP-A No. H10-104714 (JP-A-11-294244). In the device proposed herein, a solenoid valve is used to meter fuel. The opening area of this valve varies in response to changing current supplied to the solenoid. Control of this current, thereby enables precise control of the opening area of the fuel metering valve and the amount of fuel drawn into the fuel injection pump according to the operating conditions of the internal-combustion engine.
To control this current, a duty control is used (PWM control). Here, the solenoid energizing time ratio (duty ratio) per control cycle is set. The current is supplied to the solenoid for a specific time period every preset control cycle according to the duty ratio.
However, the following problems {circle around (1)} and {circle around (2)} occur if the duty control frequency is set too low (if the control cycle is set too long). Moreover, problems {circle around (3)} and {circle around (4)} arise if the control frequency is set too high (if the control cycle is set too short).
{circle around (1)} During low-frequency control (see FIG. 6A) with the duty control frequency set low, the amplitude of the current supplied to the solenoid (solenoid current) is larger than that for the high-frequency control (see FIG. 6B). Therefore, as shown in FIGS. 6C and 6D, the valve body of the fuel metering valve, which displaces according to solenoid current, moves unsteady as compared to the high-frequency control. The result is that the fuel quantity delivered from the fuel injection pump varies. Therefore, if the duty control frequency is set too low, a stabilized quantity of fuel is not delivered from the fuel injection pump.
{circle around (2)} Also, a mean value of solenoid current (mean current) is controlled for controlling the position of the fuel metering valve body to control the solenoid energizing time (duty ratio) per control cycle. The duty ratio is changed by calculations on the control device side which are used by the control after the completion of one duty control cycle and a transfer to the next control cycle. Therefore, a response delay occurs between the calculated duty ratio at the control device side and the reflected duty ratio of the solenoid current. In the case of low-frequency control, as shown in FIG. 7A, the time per control cycle becomes long as compared with that in the high-frequency control shown in FIG. 7B. Accordingly, the response delay time also becomes long. Therefore, if the duty control frequency is set too low, a lowered control response will result. The quantity of fuel delivered from the fuel injection pump, therefore, cannot be controlled quickly according to the operating condition of the internal-combustion engine.
{circle around (3)} In high frequency duty control, alternatively, solenoid current is controlled by controlling the solenoid energizing time (duty ratio) per control cycle. However, when the control device such as a microcomputer, having a digital circuit outputs a driving signal (drive pulse) for the duty control, the minimum amount of drive pulse change depends on the pulse output resolution of the control device. In this case, the higher the duty control frequency (equating to shorter control frequency), the more the duty ratio resolution becomes rough, resulting in a deteriorated control accuracy.
For example, if the duty control cycle is set at 10 msec., the pulse output resolution of the control device is 1 msec. Here, the duty control of the solenoid current can be performed at a resolution of 10%. However, to perform the duty control of the solenoid current at the control cycle of 5 msec. with the same control device, the duty control resolution will be 20%, which lowers the solenoid current control accuracy.
Therefore, if the duty control frequency is set too high when using a microcomputer (which is generally used as a control device) in the fuel injection system, the control accuracy of the solenoid current (accordingly, amount of fuel delivered from the fuel injection pump) is lowered.
{circle around (4)} If the control frequency is set too high during duty control, hysteresis results. Here, as shown in FIG. 8B, increase in valve opening (lift) relative to duty ratio (duty) change differs from decrease in valve opening (lift) relative to duty ratio change (duty) during the closing of the fuel metering valve. Therefore, it is impossible to unequivocally control the opening area (and accordingly the quantity of fuel delivered from the fuel injection pump) during the opening and closing of the fuel metering valve despite using the same duty ratio.
To accurately control solenoid current with duty control at a constant control frequency, the duty control must be carried out at such a low frequency that no hysteresis occurs between valve opening and closing. Therefore, it is necessary to set the duty control frequency so that the problems {circle around (1)} to {circle around (4)} do not occur. Therefore, conventionally, the duty control frequency is set at the optimum value applicable to the operation characteristics of the fuel injection system being controlled.
It is, however, difficult to adapt the duty control frequency to the optimum value under all operating conditions for the fuel injection system being controlled. The control characteristics vary depending on the type of control the designer believes important when setting the control frequency. That is, when the control frequency is set with importance placed on steady state control (control stability), good control response is not achieved. Likewise, when the control frequency is set with importance placed reversely on transient operation control (control response), the control stability is sacrificed.
To prevent control accuracy deterioration caused by the hysteresis phenomenon stated in {circle around (4)}, JP-A Nos. S57-157878 (JP-A-57-157878) and S62-165083 (JP-A-62-165083) disclose the solenoid energizing time or de-energizing time per control cycle is secured by changing the control frequency according to the duty ratio of the drive pulse when performing the duty control of the solenoid current. Specifically, the control frequency is lowered during a small or large duty ratio, thereby preventing the hysteresis phenomenon. In the variable control of the control frequency, the control frequency is unequivocally set in accordance with the duty ratio in either of the steady-state operation control and the transient operation control. It is therefore impossible to gain both control response and control stability. The present invention was developed in light of these drawbacks.
It is therefore an object of the present invention to ensure both steady-state operation control (control stability) and transient operation control (control response) to provide optimum fuel delivery control from the fuel injection pump at all times.
The present achieves these and other objects by providing a fuel injection system having a fuel injection pump which pressurizes fuel from a feed pump to generate high pressure fuel. The fuel injection pump delivers the high-pressure fuel to an internal-combustion engine. A fuel metering valve is provided which includes a solenoid valve and has an opening area that varies with an amount of current supplied to the solenoid valve. The fuel metering valve controls a pressure of the high-pressure fuel being delivered from the feed pump. A control means is provided for duty controlling the amount of current supplied to the solenoid valve of the fuel-metering valve so that a target state of fuel being delivered from the fuel injection pump is controlled according to an operating condition of the internal-combustion engine. The control means has a control frequency changing means that changes the duty control frequency according to the operating condition of the internal-combustion engine.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.